At the moment, there are zero airlines flying to Ulaanbaatar. Go through the results to find the right flights for you – you can adjust the filters to see find even more flights to Ulaanbaatar.
If you can't find a flight time that suits you, why not look at flying out from a different airport?
The distance between London Stansted to Ulaanbaatar is 6929 km. You can see the various flight times in the results.
Searching by month, for instance, will show you the best time of the week to fly.